Laminar-turbulent transition in Poiseuille pipe flow subjected to periodicperturbation emanating from the wall. Part 2. Late stage of transition

Transition in a fully developed circular pipe flow was investigated experim entally by introducing periodic perturbations. The simultaneous excitation of helical modes having indices m = +/-1,+/-2 and +/-3 was chosen. The expe riments revealed that the late stage of transition is accompanied by the fo rmation of streaky structures that are associated with peaks and valleys in the azimuthal distribution of the streamwise velocity disturbance. The bre akdown to turbulence starts with the appearance of spikes in the temporal t races of the velocity. Spectral characteristics of these spikes and the dir ection of their propagation relative to the wall are similar to those in bo undary layers. Analysis of the data suggests the existence of a high-shear layer in the instantaneous velocity profile. Additional experiments in which a very weak, steady flow was added locally to the periodic axisymmetric perturbation were also carried out. These expe riments resulted in the generation of a single peak in the azimuthal distri bution of the disturbance amplitude. The characteristics of the transition process (spikes, vortical patterns etc.) within this peak were similar to o nes observed in the helical excitation experiments. Based on these results one may conclude that late stages of transition in a pipe flow and in a bou ndary layer are similar. The present report is part of an ongoing investiga tion that was initiated by Eliahou, Tumin & Wygnanski (1998a).